ML19317G746
| ML19317G746 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco, Crane |
| Issue date: | 11/18/1974 |
| From: | BABCOCK & WILCOX CO. |
| To: | |
| Shared Package | |
| ML19317G737 | List: |
| References | |
| NUDOCS 8004010576 | |
| Download: ML19317G746 (5) | |
Text
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m REPORT ON INTERNALS HANDLING FIXTURE PROBLEM 7_
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This report is being made pursuant to 10 CFR 50.55(e) on significant deficien-cies to document the generic
- investigation of the significant deficiency in the design of the internals handling fixture spherical nut retaining device.
Identification of Problem During the preparation for plenum assembly removal (see Figure 1) from the j
reactor vessel at TMI-1, it was noticed that the spherical nut was riot securred to the threaded rod and could become unthreaded. This problem was identified
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through a B&W site problem report. The set screws whie.h were intended to i
prevenc relative motion between the spherical nut ar.1 the threaded rod on the three usemblies were found to be either missing, loose, or of the wrong,
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length.
Analysis I
N i
l A design review by B&W engineering has found that the sec screw concept of securring the spherical nuts from rotating relative to the threaded rods (i.e., become unthreaded) is inadequate in the original design. The spherical nuts could be unthreaded during the adjusted procedure, leaving inadequate thread engagement to support the weight of the internals.
Shou 1d the threads shear off, the resulting drop of the internals from as little as 6 inches height could result in fuel damage. Drops from greater heights could also result in damage to the internals and the reactor vessel.
Evaluation With the inadequate method of capture between the spherical nut and the threaded rod, there would be no assurance that the proper thread engagement would always be made. A small drop of % 6 inches could cause fuel assembly damage h
wnich would adversely affect the safety of the internals handling operation, and
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this design deficiency is deemed to represent a significant deficiency as o
Per 10 CFR 50.55(e).
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Corrective Actions
- 1) on TMI-1, the problem was resolved by a field modification (see Figure
- 2) which was approved by B&W engineering. This amounted to drilling a
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hole in the threaded rod to ensure capture by the set screw and staking N
of the set screw to ensure that the set screws would not back out.
- 2) A redesign of the capture device (see Figure 3) has been implemented on 5
other plants with operating license & construction permits. This design revision amounts to attaching a plate to the bottom of the threaded rod which will prohibit the spherical nut from unthr,eading
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such that the threads could not shear.
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